The invention relates generally to light-emitting diode lamps and more particularly to a light-emitting diode chip having a back reflector.
Light-emitting diode (LED) lamps utilize LED chips that are attached to lead frames which conduct excitation signals to initiate the generation of light. LEDs are well-known solid state devices that can emit light having a predefined spectral distribution. LEDs are widely used as illuminators, indicators and displays. In a typical LED lamp, the LED chip includes a layer of epoxy that bonds the LED chip to the lead frame during a die attach process.
Some types of conventional LEDs have semiconductor layers which are transparent to the emitted light and are located between the active layer and the lead frame. Such is the case for InGaN on sapphire, Transparent Substrate-AlInGaP (AlInGaP wafer bonded to a transparent GaP substrate) and GaP LEDs, which all have substrates that are transparent to the emitted light.
A concern with conventional LED lamps is that a significant amount of emitted light from the LED chip may be absorbed by the underlying material, which lowers the light output power (LOP) of the lamp. A typical method of alleviating this concern is to include silver (Ag) in the layer of epoxy on the LED chip, with the Ag functioning as a reflector. With the inclusion of Ag in the epoxy layer, some of the light that would have been absorbed by the underlying material is reflected by the Ag and is emanated from the LED chip as output light, thereby increasing the LOP of the lamp.
However, a high power LED chip recently developed by Hewlett-Packard Company, the assignee of the invention disclosed herein, requires a bonding layer and package that has a much lower thermal resistance than the epoxy/Ag layer. The high power LED chip is described in an article entitled xe2x80x9cHigh-flux high-efficiency transparent-substrate AlGaInP/GaP light-emitting diodes,xe2x80x9d by G. E. Hxc3x6fler et al., Electronic Letters, Sep. 3, 1998, Vol. 34, No. 18. The high power LED chip utilizes a layer of soldering material, instead of the epoxy/Ag layer, to bond the LED chip to a lead frame or a die pad, which also serves as a heat sink. Preferably, the soldering material has a low thermal resistance to lower the operating temperature of the LED chip, which improves light output and reliability. In addition, the high power LED chip includes a layer of reflective material to function as a reflector that increases the LOP of the lamp embodying the high power LED chip. The reflective material is chosen such that the reflector has a high reflectivity with respect to emitted wavelengths ( greater than 80%) and has a good thermal conductivity.
In FIG. 1, a high power LED chip 10 having an LED 12 and a layer 14 of soldering material is shown. The LED is an AlInGaP LED having an active layer 16, where light is generated in response to applied electrical energy. The generated light is emitted in all directions, as illustrated by the arrows near the active layer. Attached to the lower surface of the LED is ohmic contact 18. The ohmic contact is covered by an Ag reflector 20. The high power LED chip 10 also includes an ohmic contact 21, located on the upper surface of the LED. When the LED is operating, some of the light generated by the active layer of the LED propagates away from the lower ohmic contact, emanating from the LED as output light 22. However, some of the light propagates toward the ohmic contact. A portion of this light impinges the lower ohmic contact 18 and may be absorbed. Another portion of this light, however, impinges the Ag reflector, which operates to reflect the impinging light out of the chip. Thus, the intensity of the output light is increased by the light reflected from the Ag reflector.
The Ag reflector is located between the LED and the solder layer 14. The soldering material in the solder layer is indium (In). The solder layer allows the LED chip to be attached, or bonded, to an external surface (not shown).
During a high temperature process, such as a die attach process which is performed at a temperature above the melting point of the solder layer 14 (for In, the melting point is approximately 156 degrees Celsius), the Ag reflector 20 and the In solder layer 14 can intermix, reducing the reflectivity of the Ag reflector from approximately 95% to approximately less than 70%. This results in an LOP reduction of approximately 15%-20% in packaged devices.
Therefore, what is needed is a solderable high power LED chip that includes a reflector that results in high reflectivity, even after the LED chip has been subjected to high temperature processes, such as the die attach process.
A solderable light-emitting diode (LED) chip and a method of fabricating an LED lamp embodying the LED chip utilize a diffusion barrier that prevents appreciable intermixing of two different layers of the LED chip during high temperature processes. The diffusion barrier is formed of a material that appreciably blocks migration between the two layers of concern when the layers are subjected to an elevated temperature. In the preferred embodiment, the two different layers of the LED chip are a back reflector and a solder layer. By preventing the intermixing of the materials of the back reflector and the solder layer, the diffusion barrier functions to impede degradation of the back reflector with respect to its ability to reflect light emitted by the LED. The diffusion barrier should block migration into and intermixing of the solder layer into the reflector such that the reflectivity of the reflective layer is not appreciably reduced (i.e., more than 10% decrease in reflectivity) at the surface between the reflector and the LED chip. The diffusion barrier should maintain structural integrity, i.e. remain a barrier to diffusion, even at the elevated temperatures necessary to melt the solder layer.
In a first embodiment of the invention, the LED chip includes a high power AlInGaP LED. However, the type of LED included in the LED chip is not critical to the invention. Any LEDs with semiconductor layers transparent to the emitted light, located between the active layer and the solder, would benefit from this invention. Attached to the back surface of the LED is a back reflector. The back surface is the surface opposite to the light-emitting surface of the LED. Preferable for AlInGaP LEDs, the back reflector is composed of silver (Ag) or Ag alloy that has been sputtered on the back surface of the LED. However, the reflector should be optimized for the wavelength of the emitted light. Good reflectors will have reflectivity  greater than 90%. Examples of other reflectors include Al or Ag for AlGaN and Au for transparent substrate-AlGaAs LEDs. The back reflector may be formed by evaporation, electroplating, or other suitable techniques. Situated adjacent to the back reflector is the diffusion layer. In this embodiment, the diffusion layer is made of nickel (Ni) or nickel-vanadium (NiV). If the back reflector is sputtered, NiV is preferred over Ni since the NiV can be sputtered on the back reflector. The use of a sputtering process to form the diffusion barrier allows the back reflector and the diffusion barrier to be formed in a single fabrication system. Alternatively, the diffusion barrier may be formed by evaporating or electroplating Ni.
The LED chip also includes a solder layer that is affixed to the diffusion layer, such that the back reflector and the solder layer are separated by the diffusion layer. The solder layer may be made of indium (In), lead (Pb), gold (Au), tin (Sn) or their alloy and eutectics. The solder layer allows the LED chip to be mounted on an integrated heat sink, also known as the slug, or a die pad during a die attach process. The die attach process involves melting the solder layer of the LED chip to physically bond the LED chip to the slug or the die pad. However, the die attach process involves exposing the back reflector, the diffusion barrier and the solder layer to a temperature above the melting point of the soldering material (for In, the melting point is approximately 156 degrees Celsius). The diffusion barrier prevents intermixing of the In of the solder layer with the Ag of the back reflector during this high temperature process. This feature of the diffusion barrier prevents contamination of the back reflector, thereby protecting the back reflector from degradation of its high reflective characteristic. In addition to preventing intermixing of the materials, the diffusion barrier prevents degradation of the Ag in the reflector upon exposure to air.
The thickness of the diffusion barrier should be approximately 500-20,000 Angstroms. The preferred thickness of the diffusion barrier is approximately 2,000-15,000 Angstroms for Ni and approximately 1,000-10,000 Angstroms for NiV. The lower limit of the preferred thicknesses is chosen to ensure that the diffusion barrier can effectively prevent the In/Ag intermixing, while the upper limit is chosen to prevent delamination and reliability issues caused by excess stresses in the film. The difference in the preferred thickness of the Ni and NiV is due to differences in grain size, grain shape and stresses of the films.
In a second embodiment of the invention, the LED chip includes the same components of the LED chip in accordance with the first embodiment. However, in the second embodiment, the diffusion barrier is made of titanium-tungsten-nitride (TiW:N), instead of Ni or NiV. Similar to the Ni or NiV diffusion barrier of the first embodiment, the TiW:N diffusion barrier prevents intermixing of the In of the solder layer with the Ag of the back reflector and also prevents degradation of the Ag in the back reflector upon exposure to air.
Refractory metals, such as molybdenum (Mo), tungsten (W) and tantalum (Ta) may also be used to form a diffusion barrier. However, the melting point of these materials is very high (Tm=2610, 3410 and 2996xc2x0 C., respectively, compared to Ni, Tm=1455xc2x0 C.) and therefore, they are more difficult to deposit and cannot be evaporated.
In an alternative embodiment, the diffusion barrier is made of a non-conductive material. In this embodiment, the diffusion barrier may be made of dielectrics, such as aluminum-oxide (AlyOx), silicon-oxide (SiOx), silicon-nitride (SiNx), or silicon-oxygen-nitride (SiOxNy). Dielectrics are materials that have much higher resistivities (xcfx81) than metals or semiconductors (for example, for Al2O3xcfx81=1xc3x971011 ohm-cm, whereas for Si and Ni xcfx81=3 ohm-cm and 6xc3x9710xe2x88x926 ohm-cm, respectively). The resulting diffusion barrier forms a non-conductive barrier between the reflector and the solder layer that inhibits flow of electricity, in addition to preventing the intermixing of the reflective material of the reflector and the soldering material of the solder layer. However, thermal resistance of the LED is still important and should not appreciably increase with the addition of the diffusion barrier. A good estimate is not more than a 10% increase in the thermal resistance of the packaged LED should be seen with the addition of the barrier layer.
The non-conductive diffusion barrier is applicable to an LED structure that contains layers transparent to emitted light and located between the active layer and the solder layer, but does not need to conduct electricity. Such is the case for InGaN LEDs grown in sapphire. A non-conductive barrier will have resistivity many times larger than the resistivity for conductive materials. For instance, the resistivity for aluminum oxide (Al2O3) and silicon oxide (SiOx) are 3xc3x971019 and 1xc3x971021 xcexcohm-cm, respectively, compared to Ni which has a resistivity of 8 xcexcohm-cm. The resistivities for the refractory metals are much lower than those for the other non-conductive barriers listed above. Even LEDs that do not require current conduction through the barrier, however, may also utilize conductive diffusion barriers, thereby increasing the choice of materials that can be used to form the diffusion barrier.
A method of fabricating a high power LED lamp in accordance with the invention includes a step in which a layer of reflective material is deposited over the back surface of an LED. The layer of reflective material forms the back reflector of the LED chip that will be embodied in the lamp. In the preferred embodiment, the reflective material is Ag or Ag alloy, which may be sputtered on the back surface of the LED. However, other comparable reflective material may be used to form the back reflector. Next, a diffusion barrier is formed over the back reflector, such that the back reflector is positioned between the LED and the diffusion barrier. In one embodiment, the diffusion barrier is composed of Ni or NiV. In an alternative embodiment, the diffusion barrier is composed of TiW:N. A layer of soldering material is then deposited on the diffusion barrier, so that the back reflector is physically separated from the layer of soldering material by the diffusion barrier. The soldering material may be indium (In), lead (Pb), gold (Au), tin (Sn), or their alloy and eutectics. Next, the LED chip with the back reflector, the diffusion layer and the layer of soldering material is placed on a die platform, such as a slug, a die pad, or a lead frame, such that the layer of soldering material is in contact with the die platform. The LED chip is then attached to the die platform by melting the layer of soldering material, so that the soldering material bonds to the surface of the slug. After the LED chip has been mounted on the die platform, other conventional fabrication steps are performed to complete the high power LED lamp.
An advantage of the invention is that a 13-21% increase in the light output power of the LED lamp can be achieved by including the diffusion barrier in the LED chip. Furthermore, the diffusion barrier can be formed using the same sputtering or evaporation process that is utilized to form the back reflector.